The present invention relates to novel methods of measuring the parameters of twisted nematic liquid crystal devices including the twist angle, twist sense and retardation value (defined as the product of the cell gap and the liquid crystal birefringence). Apparatus for performing these measurements are also disclosed.
Conventional liquid crystal displays, including twisted nematic (TN) and super-twisted nematic (STN) LCDs, have several important design parameters, including the twist angle xcfx86 of the liquid crystal alignment and the retardation value which is defused as a product of the cell gap d and the birefringence xcex94n of the liquid crystal material (dxcex94n). These two parameters are important in determining the optical performance of the liquid crystal display. Thus the ability to determine these two parameters is important both for product development and for quality assurance.
There have been several measurement methods proposed and there exists also commercially available equipment. The measurement methods can be divided into two categories, those that require spectral measurements and those employing monochromatic light sources. All that methods use the Jones matrix as the mathematical tool. Commercially available equipment generally use the spectral method. Such machines can measure twist angle, twist sense and retardation of the LCD without any prior knowledge of the LCD rubbing directions. Monochromatic light source methods reported previously are more limited and can only measure cell gap of liquid crystal cells with known twist angles. A wave-plate compensation method can also be used to measure both the twist angle and the cell gap of an unknown LCD. Also proposed is a method for determining TN structure without using the wave-plane, this uses a rotating analyzer method. However the twist sense cannot be determined.
The present invention provides methods that can give the twist angle retardation and twist sense of any unknown LCD without using any wave-plate and without any a priori knowledge of the LCD rubbing conditions. It uses a monochromatic light source and is therefore easy to implement and economical to setup.
According to the present invention there is provided method of obtaining the twist angle and retardation value of a liquid crystal cell comprising, locating the liquid crystal cell between a first polarizer and a second polarizer serving as an analyzer, providing an input monochromatic light source and means for measuring the light transmitted through the analyzer, and:
(a) fix the polarizer-analyzer angle xcex1, at any fixed angle,
(b) rotate the liquid crystal cell while recording the light transmission
(c) note the maximum (Ki(xcex10+K2) and minimum (K1(xcex10)) values of the transmission from step (b), and
(d) solve equation                     K        1            ⁢              (                  α          0                )              =                  {                              cos            ⁢                          xe2x80x83                        ⁢            β            ⁢                          xe2x80x83                        ⁢            cos            ⁢                          xe2x80x83                        ⁢                          (                              φ                -                                  α                  0                                            )                                +                                    sin              ⁢                              xe2x80x83                            ⁢              β              ⁢                              xe2x80x83                            ⁢                              sin                ⁡                                  (                                      φ                    -                                          α                      0                                                        )                                                                                    1                +                                  r                  2                                                                    }            2                  K      2        =                            r          2                          1          +                      r            2                              ⁢              sin        2            ⁢      β      
simultaneously to obtain (xcfx86, dxcex94n) either graphically or numerically.
According to the present invention throe is further provided a method of obtaining the twist angle and retardation value of a liquid crystal cell comprising, locating the liquid crystal cell between a first polarizer and a second polarizer serving as an analyzer, providing an input monochromatic light source and means for measuring the light transmitted through the analyzer, and;
(a) fixing the polarizers in the characteristic directions of said cell whereby the angle (xcexa8) between said directions is then determined,
(b) rotate the liquid crystal cell while recording the light transmitted through the analyzer,
(c) obtain the transmission minimum (1xe2x88x92K2) and then solve equations             K      2        =                            r          2                          1          +                      r            2                              ⁢              sin        2            ⁢      β                          φ                                            δ              2                        +                          φ              2                                          ⁢      tan      ⁢                                    δ            2                    +                      φ            2                                =          tan      ⁢              (                  φ          -          ψ                )            
simultaneously to obtain (xcfx86, dxcex94a) graphically or numerically.
According to the present invention there is further provided a method of obtaining the twist angle and retardation value of a liquid crystal cell comprising locating said liquid crystal cell between a horizontal polarizer and a Stokesmeter and providing an input source of monochromatic light, and
(a) rotating the liquid crystal cell while recording the Stokes parameters, and
(b) obtaining the S1 maximum and minimum and then solve equations                     K        1            ⁢              (                  α          0                )              =                  {                              cos            ⁢                          xe2x80x83                        ⁢            β            ⁢                          xe2x80x83                        ⁢            cos            ⁢                          xe2x80x83                        ⁢                          (                              φ                -                                  α                  0                                            )                                +                                    sin              ⁢                              xe2x80x83                            ⁢              β              ⁢                              xe2x80x83                            ⁢                              sin                ⁡                                  (                                      φ                    -                                          α                      0                                                        )                                                                                    1                +                                  r                  2                                                                    }            2                  K      2        =                            r          2                          1          +                      r            2                              ⁢              sin        2            ⁢      β      
to obtain (xcfx86, dxcex94n) through the equation             I      11        =                  1        +                  S          1                    2        ,      xe2x80x83    ⁢  or
(c) obtain the S2 maximum and minimum and then solve equations             K      2        =                            r          2                          1          +                      r            2                              ⁢              sin        2            ⁢      β                          φ                                            δ              2                        +                          φ              2                                          ⁢      tan      ⁢                                    δ            2                    +                      φ            2                                =          tan      ⁢              (                  φ          -          ψ                )            
to obtain (xcfx86, dxcex94n) through the equation             I      45        =                  1        +                  S          2                    2        ,      xe2x80x83    ⁢  or
(d) obtain the characteristic angle xcexa8 and the maximum |S3| and then solve the equations                     φ                                            δ              2                        +                          φ              2                                          ⁢      tan      ⁢                                    δ            2                    +                      φ            2                                =          tan      ⁢              (                  φ          -          ψ                )                        S      3        =                  ±        J            /              cos        ⁢                  (                      φ            -            ψ                    )                    
simultaneously to obtain (xcfx86, dxcex94n).
According to the present invention there is further provided a method of obtaining the average tilt angle of a liquid crystal cell by a crystal rotation method comprising locating said liquid crystal cell between a horizontal polarizer and a Stokesmeter and providing an input source of monochromatic light, and
(a) rotating the liquid crystal cell while recording the Stokes parameters,
(b) measure the retardation maximum against rotation angle is to calculate the avenge tilt angle.
According to the present invention there is further provided a method of obtaining the characteristic directions of a liquid crystal cell comprising locating the liquid crystal cell between a first polarizer and a second polarizer serving as an analyzer, providing an input monochromatic light source and means for measuring the light transmitted through the analyzer, and:
(a) aligning the analyzer to be in the same direction as the polarizer,
(b) rotating the liquid crystal cell to obtain a transmission maximum wherein the input polarizer bisects the two characteristic directions,
(c) with the polarizer, the bisector and the analyzer aligned in the same orientation rotate the liquid crystal cell and analyzer with a magnitude ratio of 1:2 until maximum transmission is obtained wherein the polarizer and analyzer are in the primary and secondary characterization directions respectively.
According to the present invention there is further provided a method of obtaining the characteristic directions of a liquid crystal cell comprising locating the liquid crystal cell between a polarizing beam splitter and a polarizer serving as an analyzer, providing an input monochromatic light source and a first photodetector for measuring the light transmitted through the analyzer and a second photodetector for detecting light reflected from the liquid crystal cell, and:
(a) rotate the liquid crystal cell until the second photodetector measures zero intensity wherein the input polarization is in the primary characteristic direction;
(c) fix the liquid crystal cell and then rotate the analyzer to obtain a maximum first photodetector reading wherein the analyzer is in the secondary characteristic direction.
According to the present invention there is further provided a method of obtaining the retardation value of a reflective liquid crystal comprising locating the liquid crystal cell between a polarizing beam splitter and a polarizer serving as an analyzer, providing an input monochromatic light source and a first photodetector for measuring the light transmitted through the analyzer and a second photodetector for detecting light reflected from the liquid crystal cell, and:
(a) rotating the reflective liquid crystal cell until the second photodetector measures zero intensity,
(b) obtaining the retardation value can be obtained by solving the equation             φ                                    δ            2                    +                      φ            2                                ⁢    tan    ⁢                            δ          2                +                  φ          2                      =      tan    ⁢          xe2x80x83        ⁢    2    ⁢          xe2x80x83        ⁢          α      *      
with the twist angle and rubbing direction known in advance.
According to another aspect of the present invention there is provided apparatus for measuring the twist angle and retardation value of a liquid crystal cell comprising a monochromatic light source, an input polarizer, a rotary stage for the a liquid crystal cell under test, a rotary stage for an output analyzer, a photodetector to detect the transmission intensity, and a computer for controlling the rotary stages, receiving data from the photodetector and being programmed to solving the equations:                     K        1            ⁡              (                  α          0                )              =                  {                              cos            ⁢                          xe2x80x83                        ⁢            β            ⁢                          xe2x80x83                        ⁢                          cos              ⁡                              (                                  φ                  -                                      α                    0                                                  )                                              +                                    sin              ⁢                              xe2x80x83                            ⁢                              βsin                ⁡                                  (                                      φ                    -                                          α                      0                                                        )                                                                                    1                +                                  r                  2                                                                    }            2        ,      
    ⁢            K      2        =                            r          2                          1          +                      xe2x80x83                    ⁢                      r            2                              ⁢              sin        2            ⁢      β        ,            xe2x80x83        ⁢          xe2x80x83        ⁢            and      ⁢              xe2x80x83            ⁢              φ                                            δ              2                        +                          φ              2                                          ⁢      tan      ⁢                                    δ            2                    +                      φ            2                                =                  tan        ⁡                  (                      φ            -            ψ                    )                    .      
According to the present invention there is further provided apparatus for measuring the twist angle and retardation value and tilt angle of a liquid crystal cell comprising a monochromatic light same, an input polarizer, a rotary stage for the liquid crystal cell under test, a Stokesmeter to measure the output light Stokes parameters and a computer for controlling the rotary stages, receiving data from the Stokesmeter and for solving the equations:                               K          1                ⁢                  (                      α            0                    )                    =                        {                                    cos              ⁢                              xe2x80x83                            ⁢              β              ⁢                              xe2x80x83                            ⁢                              cos                ⁢                                  (                                      φ                    -                                          α                      0                                                        )                                                      +                                          sin                ⁢                                  xe2x80x83                                ⁢                                  βsin                  ⁢                                      (                                          φ                      -                                              α                        0                                                              )                                                                                                1                  +                                      r                    2                                                                                }                2              ,          
        ⁢                  K        2            =                                    r            2                                1            +                          r              2                                      ⁢                  sin          2                ⁢        β              ,          
        ⁢                  I        11            =                        1          +                      S            1                          2              ,          xe2x80x83        ⁢                            φ                                                    δ                2                            +                              φ                2                                                    ⁢        tan        ⁢                                            δ              2                        +                          φ              2                                          =              tan        ⁡                  (                      φ            -            ψ                    )                      ,          
        ⁢                  I        45            =                        1          +                      S            2                          2              ,          xe2x80x83        ⁢    and              S      3        =                  ±        J            /                        cos          ⁡                      (                          φ              -              ψ                        )                          .            
According to the present invention there is further provided apparatus for measuring the twist angle and retardation value of a liquid crystal cell comprising a monochromatic light source, an input polarizing beam splitter, a rotary stage for the liquid crystal cell under test, a partial mirror for characteristic angle setting, a rotary stage for the output analyzer, a first photodetector to detect the transmission intensity, a second photodetector to detector the reflected light intensity, and a computer for controlling the rotary stage, receiving data from the photodetectors and for solving the equations:                               K          1                ⁢                  (                      α            0                    )                    =                        {                                    cos              ⁢                              xe2x80x83                            ⁢                              βcos                ⁢                                  (                                      φ                    -                                          α                      0                                                        )                                                      +                                          sin                ⁢                                  xe2x80x83                                ⁢                β                ⁢                                  xe2x80x83                                ⁢                                  sin                  ⁢                                      (                                          φ                      -                                              α                        0                                                              )                                                                                                1                  +                                      r                    2                                                                                }                2              ,          
        ⁢                  K        2            =                                    r            2                                1            +                          r              2                                      ⁢                  sin          2                ⁢        β              ,          xe2x80x83        ⁢                            φ                                                    δ                2                            +                              φ                2                                                    ⁢        tan        ⁢                                            δ              2                        +                          φ              2                                          =              tan        ⁢                  xe2x80x83                ⁢        2        ⁢                  α          *                      ,          xe2x80x83        ⁢    and                      φ                                            δ              2                        +                          φ              2                                          ⁢      tan      ⁢                                    δ            2                    +                      φ            2                                =                  tan        ⁡                  (                      φ            -            ψ                    )                    .      